1. Field of the Invention
The present invention relates to a linear motion transmission device, and more particularly to a ball screw.
2. Description of the Prior Art
A conventional ball screw disclosed in JP Pat. No. 5-27408 comprises a screw, a nut, return assemblies, a chain and a plurality of rolling elements. The screw is defined with a helical groove, and the nut is inserted on the screw and defined with a helical groove corresponding to the helical groove of the screw. The return assemblies are disposed on the outer periphery of the nut and each provided with a return path for connecting with the helical grooves to form a circulation path. The chain is provided for holding the rolling elements and disposed in the circulation path. The return path of each return assembly is defined with a groove for passage of a connecting portion of the chain. This conventional ball screw has the following disadvantages that need to be improved:
1. As shown in FIG. 6, the chain 6 must twist while moving in the return passage of the return assembly in order to successfully pass therethrough, and the connecting portion 61 will deform along with the twisting motion of the chain 6, which results in an interference of the connecting portion 61 with the groove 7, and the position at which the interference is most likely to occur is indicated by R since the groove 7 at the position R extends inwards with respect to the rolling elements 8 and is likely to cause interference with the connecting portion 61.
2. Since the connecting portion 61 is received in the groove 7, the groove 7 requires high machining precision, and the machining will be more difficult especially when the return assemblies are small.
3. The return passage for passage of the chain consists of a passage which is circular in cross section for passage of the rolling elements and a groove which is rectangular in cross section for passage of the connecting portion 61 of the chain 6. Namely, the return passage comprises two different geometries, and the circular passage and the rectangular groove that form the return passage are in communication with each other, so that the cross section of the return passage is not a complete circular or rectangular shape. Hence, forming such a two-geometrical return passage is relatively difficult and complicated as compared to forming a single-geometrical return passage, and this two-geometrical passage is not suitable for mass production.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.